The invention concerns a motor vehicle transmission with a main transmission part and a group transmission.
In practice, in many motor vehicle transmissions, progressively stepped gear ratio steps are preferred because of their advantages in regard to drivability. Accordingly, today, nearly all motor vehicle transmissions are progressively stepped in a 1-group construction, for instance, 5 and 6 gear transmissions made by different manufacturers. Progressive transmission staging means, in this connection, that the step-jumps become continuously smaller between the individual gears up to the higher gears and the accompanying greater speeds.
In the case of multiple group transmissions, the gear stepping can no longer be made ideally progressive over the entire ratio spectrum, since this mode of construction includes a multi-usage of individual gears, and must, therefore repeat some of the jumps between the steps. Thus, for example, in the case of 2-group transmissions with split groups, necessarily, each second gear jump must be identical. This step-jump relates exactly to the step-jump of the split group. With such a foundation, it followed that all transmission gears which were available, must also be shiftable. A concept-based disadvantage of this (6-gear+split group) is that from the standpoint of a mode of construction, this can be only offered as a fast-gear design.
By means of omitting individual gears, obviously, a kind of progressive gear staging can be arrived at, however, in this way, as a rule, the cost of construction increases, since the transmission, all together, must make more gears available than, as a matter of fact, are really useable, that is, necessary. It is this disadvantageous feature which relates to the size and the higher costs.
In the case of 2-group drives with a main transmission and an area group, today it has generally become conventional, to design easily progressive transmission stepping. Even in this case, the gear jumps, because of the construction, must repeat themselves after the shifting of the area group. The exception to this repetition is the creep gear, which can only be shifted into in the slow area group.
In the 12 and 16 gear transmissions, known today, built in the 3-group mode, wherein no intermediately placed gearing for intended progressive transmission staging is omitted, only approximate geometrical gear staging is known. On this account, certain, but not necessarily intended, deviations from the purely geometrical gear staging can arise. In practice, this only occurs within certain limits of selectable tooth count relationships of individual gear steps. Design can also be a limiting factor, for instance by means of the application of the same components as the area group or by use of definite gear-pairings.
Such a transmission, in 3-group-mode has been disclosed by DE 198 31 293, which has an essentially geometric step-jump between the individual gears. However, where the number of gear changes is not so great, for instance 12 changes, the purely geometric stepping of the transmissions in 3-group-mode of construction brings about a poorer drivability as compared to that of a similarly constructed transmission with a greater number of gear changes, for instance, 16 gears.
The purpose of the invention, is to set aside the existing disadvantages and to improve the ability of making gear changes.
In accordance with the invention, the proposal is, that in the case of a transmission for a motor vehicle with a principal transmission, which possesses a number of gear changes, and that between each gear change, a step-jump is in the gear ratio is provided, and has a split transmission, for the proportioning of the step-jump of its gear steps and possesses an area group gear drive for the extension of the entire gear ratio of its gear steps. With these named features, a slow gear ratio area and a rapid gear ratio area are formed, which allows the average step-jump of the upper gear steps to be about 0.7% smaller than would be arrived at from the theoretical geometric step-jump computed from the total gear ratio. In an advantageous embodiment, the step-jump at the shifting between the slow and the rapid area of the group transmission is at least 5% greater than that gear-jump theoretically derived from the geometrically given total gear ratio.
For a 3-group-mode transmission, the following theoretical step-jump can be derived by a strict geometrical stepping of the gears:       ϕ          theor      .        =            (                        i          Gear1                          i                      Gear            ⁢                          xe2x80x83                        ⁢            z                              )              [              1        /                  (                      z            -            1                    )                    ]      
where:
z is the number of the gear changes, less an eventually present creep gear,
igear 1 is the gear ratio of the first gear, less that of the said creep gear, and
igear z is the gear ratio of the highest gear.
Where area group shifting is employed, the step-jump can be derived immediately from the ratio between the smallest, i.e., the most rapid gear ratio with the shifted, slow gear ratio area and the greatest, i.e., the slowest gear ratio with the shifted rapid gear ratio area of the area group transmission. An example would be in the case of a 16 gear transmission, namely the 9th gear and 8th gear.       ϕ    Group    =      (                  i                              min            ⁢                          xe2x80x83                        ⁢            GP                    -          L                            i                              max            ⁢                          xe2x80x83                        ⁢            GP                    -          S                      )  
The proposed solution is based on the concept, to so select the individual gear ratios in the case of a 3-group-mode transmission which intentionally deviate from the strict geometric stepping, that a progressive gear stepping is achieved. For the variations of the individual gear ratios, the following criteria were established.
The step-jumps in the upper gears, in dependency of the number of gears, were knowingly smaller than those theoretically chosen for step-jump from a purely geometric gear stepping. Accordingly, the average step-jump of the upper gears is at least 0.7% smaller than the theoretical step-jump:             4      z        ·                  ∑                  i          =          1                          n          =                      z            4                              ⁢              (                              i                          Gear              ⁡                              (                                  z                  -                  i                                )                                                          i                          Gear              ⁡                              (                                  z                  -                  i                  +                  1                                )                                                    )              ≤      0.993    ·          ϕ              theor        .            
This is valid for z=16, 12 and 8.
This fine gear stepping is immediately enabled in the upper gears by means of an step-jump increased in size by shifting in the area group. The gear stepping is at least 5% greater than the theoretical step-jump:
xcfx86Groupxe2x89xa71,05xc2x7xcfx86theor.
The following table provides, as an example, the gear ratios and the step-jumps of such a progressive, stepped 12-gear-transmission:
From the stated values, there resulted a theoretical step-jump of 1.261. The proposed solution offered fine gear stepping in the upper driving gears as well as in the most frequently encountered constant driving area. By means of a reduced cost of construction, with less gear steps, the drivability requirements can be fulfilled. A reduced fuel consumption is also evident, in relation to a geometrically stepped transmission. In practice, the larger step-jump by means of the area group at lower driving speeds is attained without any relevant disadvantages. The difference of the individual step-jumps from gear to gear also have, in practice, especially in the case of automated transmissions, no consequential disadvantages.
This invention is especially advantageous for 12-gear-transmissions with 3-group-mode. In this way, a 12-gear-transmission designed in accord with the invention can attain the drivability of a 16-gear-transmission with a clearly reduced cost of construction.